An example of such a spectrometer arrangement is an echelle spectrometer with internal order separation. The problem underlying the present disclosure shall be explained below with reference to an echelle spectrometer.
Such a spectrometer arrangement is, for example, known from DE 10 2009 059 280 A1.
In an echelle spectrometer, gratings having a ladder-like cross-section are used. Illuminating the short facet of the step-like structure with a suitable blaze angle generates a diffraction pattern, which concentrates the diffracted intensity in high orders, e.g., in the fiftieth to one-hundredth order. High spectral resolutions can thereby be achieved with a compact arrangement. The orders can be superimposed, depending on the incident wavelengths. In the case of echelle spectrometers with internal order separation, the orders are therefore again dispersed transversely to the dispersion direction of the echelle grating to separate the various orders occurring. In this way, a two-dimensional spectrum is obtained, which can be detected using area detectors.
An echelle spectrometer with internal order separation differs from echelle spectrometers with external order separation in that, in the latter, only radiation from a small spectral range enters the spectrometer. In spectrometers with internal order separation, the spectrum is produced in the form of a two-dimensional structure in the detector plane. This structure consists of spectral sections arranged essentially parallelly to one another. The free spectral ranges of the respective diffraction orders together yield a slit-free spectrum for a specific wavelength range. The use of an area detector with a plurality of detector elements allows simultaneous detection of a large wavelength range with high spectral resolution.
As mentioned above, echelle spectrometers with internal order separation in the image plane produce a spectrum in the form of a two-dimensional diffraction order structure. The distance between two adjacent orders depends on the dispersion properties of the transverse dispersion element. If a prism is used as such, the spatial separation of the orders decreases continuously as the wavelength increases. If different wavelength ranges are sequentially acquired using the spectrometer, the slit height must be adapted to the measuring range in question to maximize light conductance on the one hand and ensure clean order separation on the other hand.
To be able to resolve the intensity components of the individual wavelengths of a radiation source using a spectrometer, the optical field to be analyzed must be spatially narrowly limited. In certain cases, the image generated with the aid of an upstream lens is small enough that the field does not need to be limited (slitless spectroscopy). In other cases, the radiation of the source is guided to the spectrometer using light guides. In this case, the exit surface of the fiber can function as a field limiter.
Very frequently, however, the light source is imaged onto a field diaphragm, the entrance slit. Slit-shaped diaphragms are often used. In the dispersion direction, the slit opening is very narrow to achieve optimum spectral resolution. In this direction, this is referred to as the slit width.
Transversely to the dispersion direction, the so-called slit height tends to be selected as large as possible to improve light throughput and thus the signal-to-noise ratio. To derive a clean spectral intensity distribution from the captured image, a clean separation of the diffraction orders on the detector is required, which limits the slit height. Furthermore, the distance between two adjacent diffraction orders is variable via the wavelength range used. Depending on the transverse dispersion element used, the order separation increases (grating as transverse disperser) or decreases (prism as transverse disperser) as the wavelength increases. The height of the entrance slit is normally determined by the distance of the most closely spaced orders detected. When using area detectors for detecting the echelle spectra, a considerable part of the detector area thus remains unused.
Different individual slits are usually pivoted into the beam path when different diaphragm sizes (with respect to slit height and/or slit width) are required. However, a variable slit is also known from U.S. Pat. No. 4,325,634. R. Vuilleumier and K. Kraiczek likewise present a variable entrance slit in the article “Variable entrance slit system for precision spectrophotometers”, Micro Electro Mechanical Systems, 1995, MEMS '95, Proceedings. IEEE, DOI 10.1109/MEMSYS.1995.472583. Usually, the slit width can be adjusted by moving two slit jaws, generally via micrometer calipers. Commercially available, for example from Newport (Newport Motor Driven Slit Assembly, M5257), are motorized slits in which the slit width can be adjusted continuously via software.